The present invention relates to a control system for an electric motor, a drive having a control system and an electric motor, and a method for controlling an electric motor.
Drives having electric motors are used, inter alia, in automation technology in order to set machine parts in motion, for instance in the case of packaging machines or in the case of transport systems. Particularly if a product or a machine element is intended to be movable over a relatively long path, linear motors comprising a stator and one or a plurality of rotors movable along the stator are used as electric motors. The rotors are driven by means of a magnetic field which is generated by drive coils through which current flows, said magnetic field interacting with permanent magnets of the motor. In this case, either the drive coils or the permanent magnets are arranged on the stator and the respective other drive elements are arranged on the rotors.
In automation technology, inter alia, drives are often embodied as position-controlled drives or servo drives. In this case, the drive has a control system that continuously detects the position of the rotor along the stator. The position of the rotor is controlled to a predefined value during operation by the control system on the basis of the detected position by means of a closed control loop. For this purpose, the control system generally comprises a position detection device and an operating device. A position detection device suitable for a linear motor is disclosed in the document DE 102012204917 A1, inter alia. Programmable logic controllers or industrial PCs are often used as operating devices.
In order to ensure safe operation of machines with such drives, those movements which constitute a safety risk must be reliably prevented. Such a safety risk may reside, inter alia, in an endangerment of operating or maintenance personnel or in a movement that leads to damage to the machine, as may be the case for instance when colliding with a stop without any braking. Guiding principles of design for safety-related parts of machine controllers are predefined by international standards, inter alia. Depending on extent of damage and frequency of damage, safety-critical states or systems which prevent the occurrence of such states are assigned to a safety integrity level (SIL) or performance level (PL).
In order to enable safe operation of the drive, a machine assigned to the drive is generally monitored in a safety-relevant area, also called protection area, by means of safety sensors. Light barriers, switching contacts on guard doors, contact mats or camera systems are used, inter alia, as safety sensors. If a safety-critical state, for instance the entry of a person into the protection area, is discovered by means of the safety sensors, the drive is immediately switched safely to have no energy or no current, with the result that the drive can no longer exert any force. Suitable functions for stopping machine drive elements are internationally standardized.
What is disadvantageous about the drive being immediately switched to have no current is that, after a safety-critical state has occurred, the drive and thus the associated machine are deactivated and the work sequence is thus interrupted. In particular, it is not possible to define safe operating states in which although the drive is still supplied with energy, operating parameters of the drive do not exceed predefined safety limits.